1. Field of the Invention
This invention relates to an image forming device, and more particularly to an image forming device of a laser printer, a copying machine, or the like, which has a clamshell type frame.
2. Description of the Related Art
Some image forming devices of laser printers, copying machines, etc. have a clamshell type frame in a structure which can be opened and closed around a horizontal axis. Image forming devices of this type have upper and lower frames, and the upper frame is supported openably by the supporting shaft of the lower frame. The upper frame includes an optical unit for emitting a scanning light. The lower frame has mounted detachably thereto a process unit comprising a photosensitive body and a developing device. The upper frame has a bottom face, and in this bottom face there is formed an exposure port for emitting the scanning light from the optical unit for exposure of the photosensitive body.
In the image forming device, an electrical interlock mechanism is provided, such as a safety switch, which prevents the optical unit from operating when the upper frame is opened, and also a mechanical light shielding mechanism is provided which enables to shield light from the optical unit when the optical unit is put into operation mistakenly by an electrical trouble of the above-mentioned interlock mechanism.
This light shielding mechanism comprises a shield plate having a L-like cross section, which is rotatably supported at one end on a fixed shaft in the upper frame, and is arranged such that the light shield plate closes the exposure port by an urging force of a spring when the upper frame is opened. The fact that the exposure port is closed when the upper frame is opened not only ensures that the light is actually blocked when the optical unit is put into operation by mistake, but also gives the user a sense of security that the scanning light is not irradiated.
A working lever is provided, protruding upward, at the top of the process unit in the lower frame, which lever is used to turn the light shield plate about the fixed shaft in the printing condition that the upper frame is closed. At the bottom face of the upper frame, there is provided an opening for the working lever to enter the upper frame and comes into contact with the light shield plate when the upper frame is closed. Thus, when the upper frame is closed, the working lever contacts the light shield plate at the working point, and the light shield plate is turned about the fixed shaft to open the exposure port.
In the light shielding mechanism described, the opening angle, hence, the turning angle .theta. of the light shield plate is determined by the length and working point of the working lever. Desirably, the opening angle is sufficiently large. To this end, it is necessary to set the working point close to the exposure port or set the working point away from the exposure point and make the working lever larger accordingly.
However, making a larger working lever results in the process unit becoming difficult to handle, and this is not desirable in view of the fact that the process unit is treated as an item to be maintained by the user.
Setting the working point close to the exposure port means setting the working lever close to the photosensitive body, and therefore, the opening through which the working lever goes into and comes out of the upper frame becomes close to the photosensitive body. For this reason, through this opening, scattering toner is likely to enter the upper frame, making the optical unit dirty with toner, which has been a problem.